Method of fabricating polysilicon film

ABSTRACT

Method of fabricating polysilicon film includes forming insulating layer, first amorphous silicon layer, and cap layer over a substrate. An annealing is performed to transform the first amorphous silicon layer into first polysilicon layer with at least a hole. The cap layer is removed. A portion of the insulating layer within the hole is removed to form first opening within the insulating layer. The hole and the first opening constitute a second opening. A dielectric layer is formed over the first polysilicon layer. The dielectric layer also fills the second opening, causing a recess on the dielectric layer above the second opening. A second amorphous silicon layer is formed over the dielectric layer. A second annealing is performed to transform the second amorphous silicon layer into a second polysilicon layer. The second opening induces a thermal difference so as to cause a crystallizing direction for the second amorphous silicon layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of Ser. No. 10/709,038,filed Apr. 8, 2004, which claims the priority benefit of Taiwanapplication serial no. 92120193, filed Jul. 24, 2003, and is nowallowed. The entirety of each of the above-mentioned patent applicationsis hereby incorporated by reference herein and made a part of thisspecification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of fabricating Thin Film TransistorLiquid Crystal Display (TFT-LCD), and more particularly, relates to amethod of fabricating a polysilicon film of TFT array in a TFT-LCDthereof.

2. Description of the Related Art

An ordinary active TFT LCD array is generally categorized intopolysilicon TFT and amorphous silicon TFT based materials used formaking the TFT LCD, where a polysilicon (poly-Si) TFT being capable ofintegrating driving circuit thus provides a higher opening rate andlower fabrication cost than a corresponding amorphous silicon (a-Si)TFT. Another reason that polysilicon TFT technology is greatly promotedis that poly-Si TFT significantly reduces device feature size so thathigh image resolution can be achieved. In order to mass-producepolysilicon TFT-LCD, three primary conditions are low temperature (about450 to 550° C.) process, low-temperature filming technology for highquality gate-insulator layer, and broad ion-implantation.

In view of the cost of a glass substrate, low temperature thin filmprocess is adopted where Solid Phase Crystallization (SPC) is introducedthereby, yet the active temperature not only tends to be relativelyhigher than expected, which is around 600° C., but also causes degradedcrystallization. Thus Excimer Laser Crystallization (ELC) or ExcimerLaser Annealing (ELA) process that is applied to the foregoinglow-temperature TFT process is developed, wherein an a-Si thin film isfused by laser scanning and is crystallized to poly-Si thin film.

Providing process temperature lower than 450° C. in ELC and providinghigher electron mobility and lower current leakage than SPC in formingan amorphous silicon thin film, a less expensive glass substrate isintroduced so as to reduce fabrication cost whereas better TFT devicecharacteristic is obtained thereby.

Referring to FIG. 1A, a substrate 100 is provided. A first insulatinglayer 102 is formed on the substrate 100. Next, a photolithographyetching is performed to form a first opening 104 in the first insulatinglayer 102. In the sub-micron technology, the photolithography technologyis not applicable to the present micro TFT field, because the thresholdfeature of the first opening 104 using photolithography technique isabout 1 micrometer, which is relatively large compared to the thresholdcrystal feature size for TFT thin film.

Attempts to resolve the issue is illustrated with reference to FIG. 1B.A second insulating layer 106 is further formed over the firstinsulating layer 102 and the first opening 104. The deposition of thesecond insulating layer 106 further shrinks the first opening 104 to asecond opening 108 to satisfy the feature size requirement forpolysilicon TFT crystallization.

Referring to FIG. 1C, an a-Si layer 110 is formed over the secondinsulating layer 106. Next, fuse and liquefy the a-Si layer 110 by anExcimer Laser 112.

Finally, referring to FIG. 1D, the fused liquefied silicon undergoescrystallization from the second opening 108 to transform the a-Si layer110 into a poly-Si layer 114, which is suitable for forming source/drainand channel of a TFT therein.

However, problems in the foregoing process do exist, as described below.The forming of the first opening 104 in the foregoing process requires amask process and an additional deposition step of forming the secondinsulating layer 106 adjusting to the size of the first opening 104, andtherefore not only complication but also lowers throughput results.

Moreover, the scheme of depositing the second insulating layer 106 foradjusting to the size of the second opening 108 requires precise controlof the process conditions, thus narrowing the processing tolerancewindow.

SUMMARY OF INVENTION

According to foregoing issues, one object of the present invention is toprovide a method of fabricating a poly-Si thin film, wherein the stepsof complicated photolithography exposure, extra deposition procedure,etc. can be excluded, and an opening with proper deep sub-microndimensions can be formed.

Another object of the present invention is to provide a method offabricating a poly-Si film, wherein an opening having a size sufficientfor poly-Si thin film crystallization can be formed without precisecontrol of process conditions, and thereby increasing the process windowallowing greater process condition tolerance.

The present invention provides a method of fabricating a polysiliconfilm, comprising providing a substrate. An insulating layer, a firstamorphous silicon layer, and a cap layer are formed over the substrate.A first annealing is performed to transform the first amorphous siliconlayer into a first polysilicon layer with at least a hole. The cap layeris removed. A portion of the insulating layer within the hole is removedto form a first opening within the insulating layer, wherein the holeand the first opening constitute a second opening. A dielectric layer isformed over the first polysilicon layer, wherein the dielectric layeralso fills the second opening and a recess is formed over a portion ofthe dielectric layer above the second opening. A second amorphoussilicon layer is formed over the dielectric layer. A second annealing isperformed to transform the second amorphous silicon layer into a secondpolysilicon layer. Wherein, the second opening induces a thermaldifference so as to cause a crystallizing direction for the secondamorphous silicon layer.

It should be noted that, in the step of forming the dielectric layerover the first polysilicon layer, the dielectric layer solidly fillsinto a space of the second opening, or alternatively, the dielectriclayer fills into a space of the second opening with a sub-hole withinthe second opening.

According to the foregoing description, it is noted that a proper deepsub-micron hole in the insulating layer is formed by sequentiallyforming an insulating layer, a a-Si layer and a cap layer over thesubstrate and then performing a laser annealing process withoutperforming any photolithography and etching. Accordingly, process stepssuch as light exposure, photolithography and additional deposition asdescribed above for forming an opening having a deep sub-micron featurecan be effectively excluded. Thus, the throughput can also beeffectively increased.

Moreover, the method of the present invention can be implemented withoutprecisely controlling the process conditions by forming the cap layer,the a-Si layer, the insulating layer or laser annealing process. Thusthe method of the present invention has a broader process tolerancecompared to the conventional process described above.

DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D show the cross sectional views illustrating theprogression of the process according to a conventional method offabricating a polysilicon (poly-Si) thin film.

FIGS. 2A to 2E show the cross sectional views illustrating theprogression of the process of a method of fabricating a poly-Si thinfilm according to an embodiment of the present invention.

FIGS. 3A to 3F show the cross sectional views illustrating theprogression of the process of a method of fabricating a poly-Si thinfilm according to another embodiment of the present invention.

FIGS. 4A to 4F show the cross sectional views illustrating theprogression of the process of a method of fabricating a poly-Si thinfilm according to another embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Referring to FIGS. 2A to 2E, show the cross sectional views illustratingthe progression of the process of a method of fabricating a polysilicon(poly-Si) thin film according to the first embodiment of the presentinvention.

Referring to FIG. 2A, a substrate 200 is provided, wherein the materialof the substrate 200 includes a silicon wafer, a glass substrate or aplastic substrate, for example. An insulating layer 202 is formed overthe substrate 200, wherein the insulating layer 202 includes silicondioxide can be formed by performing a conventional deposition processsuch as Low Pressure Chemical Vapor Deposition (LPVCD), Plasma EnhancedChemical Vapor Deposition (PECVD) or sputtering. Thereafter a first a-Silayer 204, which can be formed by performing a conventional process suchas LPVCD, PECVD or sputtering, is formed over the insulating layer 202.

Further, a cap layer 206 is formed over the first a-Si layer 204,wherein the material of the cap layer 206 includes a silicon dioxide,for example, wherein the cap layer 206 may be formed by performing aconventional deposition process such as LPCVD, PECVD, or sputtering.Afterwards, the resulting structure is subjected to a first laserannealing 208, for example, an excimer laser may be used to perform thefirst laser annealing 208, so as to fuse the first a-Si layer 204. Theenergy density of the excimer laser is about 50 to 500 mJ/cm².

Referring to FIG. 2B, a first poly-Si layer 210 is formed transformedfrom the first a-Si layer 204 through crystallization. In addition, aplurality of holes are randomly formed in the first poly-Si layer 210,however, in the FIG. 2B, only one hole 212 is shown for illustrationpurpose.

According to the foregoing procedures, the reasons why the hole 212 isformed in the first poly-Si layer 210 is not exactly known but it ismost likely due to a cohesion force of poly-Si being stronger than anadhesion force between the cap layer and the first poly-Si layer 210.The first poly-Si layer 210 shrinks inwardly to form the holes 212 asthe first a-Si layer 204 is transformed into the first poly-Si layer210. Additionally, each of the holes 212 has the feature of a properdeep sub-micron dimension for back-end crystallization.

Referring to FIG. 2C, the cap layer 206 is removed by performing a wetetching or an anisotropic dry etching. Thereafter, a portion of theinsulating layer 202 exposed within the hole 212 is removed to form afirst opening 214, wherein the step of removing the portion of theinsulating layer 202 exposed within the first opening 214 can be carriedout by performing a wet etching, for example. The width of the firstopening 214 is smaller than about 0.5 micron for furthercrystallization. The hole 212 and the first opening 214 form a secondopening 216.

Referring to FIG. 2D, a second a-Si layer 218 is formed over the firstpoly-Si layer 210 and the second opening 216, wherein the second a-Silayer 218 is deposited by performing LPCVD, PECVD, or sputtering, forexample, wherein the second a-Si layer 218 includes a recess 220neighboring with the second opening 216. The resulting structure issubjected to a second laser annealing 222, for example, using an excimerlaser to irradiate the second a-Si layer 218 with an energy density ofabout 50 to 500 mJ/cm² so as to fuse the second a-Si layer 218 and thefirst poly-Si layer 210. According to the second opening 216, an unfusedportion of the second a-Si layer 218 serves as a seed forcrystallization, wherein the unfused portion of the second a-Si layer218 is at the bottom of the second opening 216.

Finally, referring to FIG. 2E, a second poly-Si layer 224 is transformedfrom a fused portion of the second a-Si layer 218 and the first poly-Silayer 210 crystal growing in a lateral direction 226.

Referring to the FIGS. 3A to 3F, show the cross sectional viewsillustrating the progression of the process of a method of fabricating apoly-Si film according to a second embodiment of the present invention.

Referring to FIG. 3A, a substrate 300 is provided, wherein the materialof the substrate 300 includes, for example, a silicon wafer, a glass ora plastic. An insulating layer 302 is formed over the substrate 300,wherein the material of the insulating layer 302 includes, for example,a silicon dioxide, and the insulating layer 302 can be formed by, forexample, performing a conventional deposition process such as a LPVCD, aPECVD or a sputtering. Thereafter, a first a-Si layer 304 is formed overthe insulating layer 302, by performing, for example, a LPCVD, PECVD orsputtering process.

Further, a cap layer 306 is formed over the first a-Si layer 304,wherein the material of the cap layer 306 includes, for example, silicondioxide, and the cap layer 306 can be formed by, for example, performinga conventional deposition process such as LPCVD, PECVD or sputtering.The resulting structure is then subjected to a first laser annealing308, for example, performing an excimer laser annealing to fuse thefirst a-Si layer 304. The energy density of the excimer laser is about50 to 500 mJ/cm².

Referring to FIG. 3B, a first poly-Si layer 310 is formed transformedfrom the first a-Si layer 304 through the fusion and crystallization.Moreover, as described in the first embodiment, as the first a-Si layer304 is transformed to the first poly-Si layer 310, a plurality of holes312 are randomly formed in the first poly-Si layer 310, however only asingle hole 312 is shown in FIG. 3B for illustration purpose.

Referring to FIG. 3C, the cap layer 306 is removed, wherein the step ofremoving the cap layer 306 is accomplished by, for example, performing awet etching using hydrofluoric acid or an anisotropic dry etching.Thereafter, a portion of the insulating layer 302 exposed within thehole 312 is removed to form a first opening 314, wherein the portion ofthe insulating layer 302 exposed within the first opening 314 can beremoved by, for example, performing a wet etching. The first opening 314has a width smaller than about 0.5 micron for further crystallization.The hole 312 and the first opening 314 constitute a second opening 316.

Referring to FIG. 3D, a dielectric layer 318 is formed over the firstpoly-Si layer 310 and the second opening 316, wherein the dielectriclayer 318 can be formed by, for example, performing a conventionalprocess such as either LPCVD, PECVD or sputtering, wherein thedielectric layer 318 includes a recess 320 neighboring with the secondopening 316.

Referring to FIG. 3E, a second a-Si layer 322 is formed over thedielectric layer 318, wherein the second a-Si layer 322 is formed by,for example, performing with a conventional deposition process such as aLPCVD, a PECVD, or a sputtering process. Thereafter, the resultingstructure is subjected to a second laser annealing 324 by performing,for example, an excimer laser annealing, to irradiate the second a-Silayer 322. The energy density of the excimer laser is about 50 to 500mJ/cm².

Finally, referring to FIG. 3F, a second poly-Si layer 326 is formedtransformed from a fused portion of the second a-Si layer 322 crystalgrowing in a lateral direction 328, wherein an unfused portion of thesecond a-Si layer 322 neighboring with the recess 320 serves as a seedfor crystallization.

As described above, the opening 316 is used to induce the thermaldifference, so that the a-Si layer 322 has a crystallizing direction.With the similar mechanism but in different condition, a differentcrystallizing direction can also be created. FIGS. 4A to 4F show thecross-sectional views illustrating the progression of the process of themethod of fabricating a poly-Si film according to another embodiment ofthe present invention.

Referring to FIG. 4A, a substrate 400 is provided, wherein the materialof the substrate 400 includes, for example, silicon wafer, glass, orplastic. An insulating layer 402 is formed over the substrate 400,wherein the material of the insulating layer 402 includes, for example,silicon dioxide, and wherein the insulating layer 402 can be formed byperforming conventional deposition methods such as LPVCD, PECVD, orsputtering. Thereafter a first a-Si 404 is formed over the insulatinglayer 402, which can be formed by performing LPCVD, PECVD or sputteringmethod, for example.

Next, a cap layer 406 is formed over the first a-Si layer 404, whereinthe material of the cap layer 406 includes, for example, silicondioxide, and wherein the cap layer 406 can be formed by performingconventional deposition methods such as LPCVD, PECVD or sputteringmethod. Thereafter, the resulting structure is subject to a first laserannealing 408 by performing, for example, an excimer laser, so as tofuse the first a-Si layer 404. The energy density of the excimer laseris about 50 to 500 mJ/cm².

Referring to FIG. 4B, a first poly-Si layer 410 is formed from the firsta-Si layer 404 through fusion and crystallization. Moreover, a pluralityof first holes 412 are randomly formed in the first poly-Si layer 410,however, in the FIG. 4B, only one first hole 412 is shown forillustration purpose.

Further, referring to FIG. 4C, the cap layer 406 is removed, wherein themethod for removing the cap layer 406 is accomplished by performing awet etching using hydrofluoric acid or anisotropic dry etching.Thereafter, a portion of the insulating layer 402 within the first hole412 is removed to form a first opening 414, wherein the portion of theinsulating layer 402 is removed by performing a wet etching, forexample. The first opening 414 formed by the foregoing method has awidth smaller than about 0.5 micron for further crystallization. Thefirst opening 412 and the first opening 414 constitute a second opening416.

Next, referring to FIG. 4D, a dielectric layer 418 is formed over thefirst poly-Si layer 410 and the second opening 416, wherein thedielectric layer 418 can be formed by performing LPCVD, PECVD orsputtering, for example. A second hole 420 is formed as an air space inthe dielectric layer 418, wherein the second hole 420 is neighboringwith the second opening 416.

Furthermore, referring to FIG. 4E, a second a-Si layer 422 is formedover the dielectric layer 418, wherein the second a-Si layer 422 isformed by performing LPCVD, PECVD or sputtering, for example.Thereafter, the resulting structure is subjected to a second laserannealing 424 by performing an excimer laser annealing for example, toirradiate and fuse the second a-Si layer 422. The energy density of theexcimer laser is about 50 to 500 mJ/cm².

Finally, referring to FIG. 4F, a second poly-Si layer 426 is transformedfrom the second a-Si layer 422 through fusion and crystallization. Whenthe second laser annealing 424 is performed, a portion of the seconda-Si layer 422 over the second hole 420 is subjected to a highertemperature than other portion of the second a-Si player 422 relative tothe second hole 420 because the thermal conductivity is poor around thesecond hole 420. A lateral crystallization progress from a region withlowest temperature (not shown) along the direction 428 is performed,wherein the lateral crystallization lasts longer around the second hole420.

It can be seen that the foregoing embodiments are similarly using thesecond opening to induce the thermal difference of the dielectric layer,so as to further cause the crystallization direction for the second a-Silayer during annealing process.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncovers modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A method of fabricating a polysilicon film, comprising: providing asubstrate; forming an insulating layer, a first amorphous silicon layer,and a cap layer over the substrate; performing a first annealing totransform the first amorphous silicon layer into a first polysiliconlayer with at least a hole; removing the cap layer; removing a portionof the insulating layer within the hole to form a first opening withinthe insulating layer, wherein the hole and the first opening constitutea second opening; forming a dielectric layer over the first polysiliconlayer, wherein the dielectric layer fills the second opening and arecess is formed over a portion of the dielectric layer above the secondopening; forming a second amorphous silicon layer over the dielectriclayer; and performing a second annealing to transform the secondamorphous silicon layer into a second polysilicon layer, wherein thesecond opening induces a thermal difference so as to cause acrystallizing direction for the second amorphous silicon layer.
 2. Themethod of claim 1, wherein forming the dielectric layer over the firstpolysilicon layer comprises filling the dielectric layer into a space ofthe second opening.
 3. The method of claim 1, wherein forming thedielectric layer over the first polysilicon layer comprises filling thedielectric layer with a sub hole within the second opening into a spaceof the second opening.
 4. The method of claim 1, wherein the cap layercomprises a silicon dioxide.
 5. The method of claim 1, whereinperforming the first annealing process comprises performing an excimerlaser annealing process.
 6. The method of claim 1, wherein removing theportion of the insulating layer within the hole comprises performing awet etching with a solution containing hydrofluoric acid.
 7. The methodof claim 1, wherein performing the second annealing comprises performingan excimer laser annealing process.
 8. The method of claim 1, whereinthe dielectric layer comprises a silicon dioxide.
 9. The method of claim1, wherein the second opening has a width less than one micron.